Throughout the process of transforming a silicon wafer to a functional microelectronic device, there are several stages of processing that must occur. Each stage requires a specialized tool, which may require that the wafer be precisely aligned on its chuck for optimal performance.
By way of example, a complicated process using a plurality of superimposed photo-exposure masks is used to fabricate a highly integrated semiconductor device (IC). Particularly, the process includes using photo-exposure masks for each processing step. To use the photo-exposure masks, it is necessary to align such masks using a specifically shaped mark. This type of mark is called an alignment mark, i.e., a superimposition mark or an alignment key, which is essential for an overlap process. However, using known processes, it is becoming ever more difficult to properly align the wafer at each stage of the process, thus contributing to lower yields.
Moreover, as higher alignment precision is required for the manufacture of ICs in order to produce ever shrinking line widths while providing greater degree of functional integration, the difficulties to accurately and clearly detect and measure the positions of the alignment marks on a wafer often become a limitation to further miniaturize the IC devices. The difficulties arise from the fact that the alignment marks on a wafer employed for relative position measurements are often smeared or stained as the results of various manufacturing processes. And, as the alignment marks become blurred, an alignment measurement may not be accurately performed.
For example, as a CMP process is provided during the fabrication process, the fine line definition of the alignment marks may be lost and no longer suitable for the purpose of high precision alignment. Additionally, in the process of forming various circuit elements, various layers are formed over the top surface and then patterned, also causing the alignment marks to become blurred and unclear.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.